Automobile and other manufacturers are strongly demanding an electro-galvanized steel strip or an electro-zinc-alloy-galvanized steel strip (hereinafter generally referred to as "electro-galvanized steel strip") having a coating uniform in the chemical composition and thickness and having an excellent appearance.
The following electro-galvanizing apparatuses are known for electro-galvanizing at least one of the surfaces of a steel strip with zinc or electro-zinc-alloy-galvanizing at least one of the surfaces of a steel strip with alloy comprising zinc and at least one of iron, nickel, cobalt, chromium and other metals (hereinafter generally referred to as "electro-galvanizing"):
(1) a horizontal type electro-galvanizing apparatus comprising a horizontal type electro-galvanizing tank for receiving an electro-galvanizing bath and at least one electrode plate arranged in said electro-galvanizing tank substantially in parallel with a steel strip horizontally travelling through said electro-galvanizing tank, and
(2) a vertical type electro-galvanizing apparatus comprising a vertical type electro-galvanizing tank for receiving an electro-galvanizing bath, at least one first electrode plate and at least one second electrode plate arranged in said electro-galvanizing tank substantially in parallel with a steel strip vertically travelling down and then up through said electro-galvanizing tank.
As compared with the horizontal type electro-galvanizing apparatus, the vertical type electro-galvanizing apparatus has the following advantages:
(1) In the horizontal type electro-galvanizing apparatus, wavy irregularities are produced in the steel strip during travelling through the horizontal type electro-galvanizing tank, and as a result, there occur variations in the distance between the steel strip and the at least one electrode plate, thus resulting in non-uniform thickness of the coating. In addition, contact of the steel strip with the at least one electrode plate may cause damages to the steel strip. In the vertical type electro-galvanizing apparatus, in contrast, no wavy irregularities occur in the steel strip during travelling through the vertical type electro-galvanizing tank, thus permitting manufacture of an electro-galvanized steel strip free from damage, with a uniform thickness of the coating.
(2) The vertical type electro-galvanizing apparatus requires only a small site area for installation.
As a vertical type electro-galvanizing apparatus, an apparatus is disclosed in the publication entitled "Iron and Steel Handbook" (Tekko-Binran), Vol. IV, published on May 31, 1982 in Japan, which comprises:
a vertical type electro-galvanizing tank for receiving an electro-galvanizing bath; a first conductor roll, arranged above a steel strip inlet side of said electro-galvanizing tank, for substantially vertically directing a steel strip downwardly into said electro-galvanizing tank and applying electricity to said steel strip; a sink roll, arranged in said electro-galvanizing tank, for upwardly reversing the travelling direction of said steel strip directed into said electro-galvanizing tank; a second conductor roll, arranged above a steel strip outlet side of said electro-galvanizing tank, for substantially vertically directing said steel strip upwardly from said electro-galvanizing tank and applying electricity to said steel strip; a pair of first electrode plates for electro-galvanizing the both surfaces of said steel strip, which are arranged in said electro-galvanizing tank between said first conductor roll and said sink roll substantially in parallel with said steel strip travelling downwardly through said electro-galvanizing tank with said steel strip therebetween; a pair of second electrode plates for electro-galvanizing the both surfaces of said steel strip, which are arranged in said electro-galvanizing tank between said sink roll and said second conductor roll substantially in parallel with said steel strip travelling upwardly through said electro-galvanizing tank with said steel strip therebetween; an electro-galvanizing solution supply means for supplying an electro-galvanizing solution into said electro-galvanizing tank to form an electro-galvanizing bath therein, and forming a flow at a prescribed flow velocity of said electro-galvanizing bath between said pair of first electrode plates and said steel strip travelling downwardly and between said pair of second electrode plates and said steel strip travelling upwardly; and an electro-galvanizing bath discharge means, provided at a top portion of said electro-galvanizing tank, for causing overflow of said electro-galvanizing bath in said electro-galvanizing tank (hereinafter referred to as the "prior art").
FIG. 5 is a schematic longitudinal sectional view of the above-mentioned apparatus of the prior art. As shown in FIG. 5, a first conductor roll 2 for substantially vertically directing a steel strip 5 downwardly into a vertical type electro-galvanizing tank 1 and applying electricity to the steel strip 5 is arranged above a steel strip inlet side of the electro-galvanizing tank 1. A sink roll 3 for upwardly reversing the travelling direction of the steel strip 5 directed into the electro-galvanizing tank 1 is arranged in the lower portion of the electro-galvanizing tank 1. A second conductor roll 4 for substantially vertically directing the steel strip 5 upwardly from the electro-galvanizing tank 1 and applying electricity to the steel strip 5 is arranged above a steel strip outlet side of the electro-galvanizing tank 1.
A pair of first electrode plates 6, 6 and a pair of second electrode plates 7, 7 for electro-galvanizing the both surfaces of the steel strip 5 are arranged in the electro-galvanizing tank 1. The pair of first electrode plates 6, 6 are arranged between the first conductor roll 2 and the sink roll 3 substantially in parallel with the steel strip 5 travelling downwardly through the electro-galvanizing tank 1 with the steel strip 5 therebetween. The pair of second electrode plates 7, 7 are arranged between the sink roll 3 and the second conductor roll 4 substantially in parallel with the steel strip 5 travelling upwardly through the electro-galvanizing tank 1 with the steel strip 5 therebetween.
A pair of first nozzles 8, 8 and a pair of second nozzles 9, 9 for supplying an electro-galvanizing solution into the electro-galvanizing tank 1 to form an electro-galvanizing bath therein are arranged in the lower portion of the electro-galvanizing tank 1. The pair of first nozzles 8, 8 are upwardly arranged respectively below each of the pair of first electrode plates 6, 6 near the lower ends thereof so as to form a rising flow of electro-galvanizing bath at a prescribed flow velocity between each of the pair of first electrode plates 6, 6 and the steel strip 5 travelling downwardly. The pair of second nozzles 9, 9 are upwardly arranged respectively below each of the pair of second electrode plates 7, 7 near the lower ends thereof so as to form a rising flow of electro-galvanizing bath at a prescribed flow velocity between each of the pair of second electrode plates 7, 7 and the steel strip travelling upwardly. An electro-galvanizing bath discharge pipe 10 for causing overflow of the electro-galvanizing bath in the electro-galvanizing tank 1 is provided at a top portion 1a of the electro-galvanizing tank 1.
Electro-galvanizing solution ejected into the electro-galvanizing bath from each of the pair of first nozzles 8, 8 and the pair of second nozzles 9, 9 rises between each of the pair of first electrode plates 6, 6 and the steel strip 5 travelling downwardly, and between each of the pair of second electrode plates 7, 7 and the steel strip 5 travelling upwardly, and overflows through the electro-galvanizing bath discharge pipe 10. The steel strip 5 travels downwardly and then upwardly in the electro-galvanizing tank 1 through the first conductor roll 2, the sink roll 3 and the second conductor roll 4, and the both surfaces of the steel strip 5 are electro-galvanized while the steel strip 5 passes between the pair of first electrode plates 6, 6 and between the pair of second electrode plates 7, 7.
The above-mentioned apparatus of the prior art has the following problem: The flow velocity of electro-galvanizing bath which rises between the pair of first electrode plates 6, 6 and the steel strip 5 travelling downwardly and between the pair of second electrode plates 7, 7 and the steel strip 5 travelling upwardly, is reduced as the distance from the pair of first nozzles 8, 8 and the pair of second nozzles 9, 9 becomes longer, resulting in a non-uniform flow velocity of the electro-galvanizing bath. When the flow velocity of the electro-galvanizing bath rising between the pair of first electrode plates 6, 6 and the steel strip 5 travelling downwardly and between the pair of second electrode plates 7, 7 and the steel strip 5 travelling upwardly becomes non-uniform, irregularities are produced on the surfaces of the coatings formed on the surfaces of the steel strip 5, and particularly when the coating is a zinc alloy coating such as an iron-zinc alloy coating, there occur variations in the chemical composition of the zinc alloy coating.
A problem similar to that mentioned above is also encountered when arranging the pair of first nozzles 8, 8 and the pair of second nozzles 9, 9 above the pair of first electrode plates 6, 6 and the pair of second electrode plates 7, 7 to form downward flows of electro-galvanizing bath respectively between the pair of first electrode plates 6, 6 and the steel strip 5 travelling downwardly and between the pair of second electrode plates 7, 7 and the steel strip 5 travelling upwardly, or when arranging the pair of first nozzles 8, 8 and the pair of second nozzles 9, 9 on the sides of the pair of first electrode plates 6, 6 and the pair of second electrode plates 7, 7 to form flows of electro-galvanizing bath in the width direction of the steel strip 5 respectively between the pair of first electrode plates 6, 6 and the steel strip 5 travelling downwardly and between the pair of second electrode plates 7, 7 and the steel strip 5 travelling upwardly.
Under such circumstances, there is a strong demand for the development of a vertical type electro-galvanizing apparatus which can ensure a uniform flow velocity of an electro-galvanizing bath flowing respectively between at least one first electrode plate arranged in a vertical type electro-galvanizing tank and a steel strip travelling through the tank and between at least one second electrode plate arranged also in the tank and the steel strip to form a coating free from irregularities and having a uniform chemical composition on at least one of the surfaces of the steel strip, but such an apparatus has not as yet been proposed.